HA: Sensors that calculate and compare the Heat Discomfort Index for inside and outside.

nicer_outside.yaml

template:
  - sensor:
    - name: "Outside Discomfort Index"
      unit_of_measurement: ""
      state_class: measurement
      state: >-
        {% set t = ((states('sensor.outside_temp')|float(0)) - 32) / 1.8 %}
        {% set h = states('sensor.outside_humidity')|float(0) %}
        {% set discomfort_index = (t - (0.55*(1  - (0.01*h) ) * (t - 14.5)))|round(2,0) %}
        {{ discomfort_index}}
      attributes:
        comfortable: >-
          {{ this.state|int(0) < 20 }}
    - name: "Inside Discomfort Index"
      unit_of_measurement: ""
      state_class: measurement
      state: >-
        {%- set t = ((states('sensor.inside_temp')|float(0)) - 32) / 1.8 %}
        {%- set h = states('sensor.inside_humidity')|float(0) %}
        {%- set discomfort_index = (t - (0.55*(1  - (0.01*h) ) * (t - 14.5)))|round(2,0) %}
        {{ discomfort_index }}
      attributes:
        comfortable: >-
          {{ this.state|int(0) < 20 }}
  - binary_sensor:
    - name: "Nicer Outside"
        state: >-
          {% set threshold = 1.5 %}
          {{ states('sensor.inside_discomfort_index')|float(0)|round(1, default=0) - threshold
          >= states('sensor.outside_discomfort_index')|float(0)|round(1, default=0) }}

Formula for Heat Discomfort from: https://keisan.casio.com/exec/system/1351058230

The two Discomfort Index sensors are setup to convert Fahrenheit temperatures reported by my sensors to metric.
If your sensors report in Celsius, change the first line of the state template as follows:

{% set t = states('sensor.outside_temp')|float(0) %}

The threshold variable should be set to personal preference... Basically, "How much nicer does it need to be inside to count?"